This invention generally relates to electromagnetic clutches, and is particularly concerned with an improved clutch having a permanent magnet return mechanism for avoiding damage from torque oscillations.
In transport refrigeration systems, it is common to drive the refrigerant compressor from one of two available prime movers, including a diesel internal combustion engine, and a stand-by electric motor. A clutch is used to engage and disengage the refrigerant compressor from the diesel engine. In the past, centrifugal type clutches have been used for this purpose. However, in order to reduce the level of noise associated with the operation of the refrigeration system, it has been desirable in recent years to lower the operational speed of the engine. Such a lowering of operating speed renders the use of a centrifugal clutch impractical. Thus, the use of electromagnetic clutches instead of centrifugal clutches is becoming more common.
Unfortunately, the torque output of a diesel engine is variable due to the operating characteristics of such devices. For example, for a diesel engine of the type used to run a transport refrigeration system, the average torque produced might be on the order of 40 ft/lb. However, such an output may be accompanied by momentary "spikes" of torque on the order to 300 ft/lb, particularly if the diesel engine is operated at a low rotational speed. Such torsional "spikes" are capable of damaging the mechanical linkage between the flywheel of the clutch and the contact plate assembly that engages the clutch rotor.
To prevent such damage from occurring between the flywheel and the contact plate assembly, resilient linkage components have been used in the prior art. For example, in U.S. Pat. No. 5,252,874 (also assigned to Thermo King Corporation) the flywheel and the contact plate assembly are interlinked by means of a system of pins that are slidably movable in resilient bushings. The linkage disclosed in this patent further uses compression springs to bias the contact plate assembly into a non-engagement position with respect to a rotor which in turn is connected to the shaft of the compressor. When the electromagnet of the clutch is actuated, the bias afforded by the compression springs is overcome, and the contact plate assembly engages the rotor of the clutch so that power is transmitted from the output shaft of the diesel engine to the input shaft of the compressor.
While the use of resilient components in the linkage between the flywheel and contact plate assembly has been found to greatly extend the life time of the linkage, the applicants have observed that it does not completely insulate the linkage from damage caused by torsional spikes. In particular, the applicants have observed that the compression springs used to return the contact plate assembly into a non-engagement position may fail after several thousand service hours. Such failure is substantially unexpected, as it was thought that the inherent resiliency of the compression springs would isolate them from such failure.
Clearly, there is a need for an improved electromagnetic clutch having a linkage between the contact plate assembly and the flywheel which is better able to endure a power input accompanied by sharp spikes of torque of the type frequently generated by diesel engines. Preferably, the design of the improved linkage could be easily incorporated into the design of prior art devices such as that disclosed in U.S. Pat. No. 5,252,874.